The present invention relates to automotive heater systems employing heat generators that use viscous fluid to generate heat.
Heater systems are installed in automobiles to warm the passenger compartment. A heater system generally has a heater core, which is normally connected to the water jacket of an engine through a coolant circuit. The engine serves as a heat source in the heater system. When engine coolant passes through the water jacket, heat transfer occurs between the coolant and the engine. This cools the engine and heats the engine coolant. The heat carried by the engine coolant is then used to warm the passenger compartment with the heater core. However, much time is required to warm the passenger compartment, especially, when the engine is started when it is cold. Thus, heat generators serving as auxiliary heat sources have been proposed.
U.S. Pat. No. 4,993,377 describes a typical heat generator that assists the engine to heat engine coolant. The heat generator includes a housing, a viscous fluid (e.g., silicone oil) contained in the housing, and a uniquely designed rotor. The rotor, which is fixed to a drive shaft, is rotated to shear the viscous fluid and generate heat. The drive shaft is connected to an electromagnetic clutch. The electromagnetic clutch has a pulley that is connected to the engine by a belt. The drive shaft and the engine are selectively connected to and disconnected from each other by the electromagnetic clutch. A coolant circuit, through which the engine coolant flows, extends between the engine and the heat generator. The heat generated by the engine and the heat generator is carried by the engine coolant and used to warm the passenger compartment.
The heat generator is controlled by a controller (e.g., electronic control unit). The controller excites the electromagentic clutch and rotates the drive shaft and the rotor with the power of the engine to activate the heat generator. The heat generator is kept activated until the temperature of the engine coolant flowing through the coolant circuit exceeds a predetermined limit temperature, that is, until there is no need for the heat generator to further heat the engine coolant. If the engine coolant temperature exceeds the predetermined limit, the controller de-excites the electromagnetic clutch and disconnects the heat generator from the engine. In this state, the engine sufficiently warms the engine coolant and thus the passenger compartment without assistance from the heat generator.
The temperature of the viscous fluid in the heat generator normally increases as the rotating speed of the rotor, or the engine speed, increases. However, viscous fluid becomes vulnerable to thermal deterioration, which is caused by heat, and mechanical deterioration, which is caused by shearing, as the temperature of the viscous fluid exceeds a certain value. For example, when using high-viscosity silicone oil as the viscous fluid, the silicone oil becomes vulnerable to deterioration if the oil is continuously sheared at a temperature, that exceed a maximum heat-generating temperature of 200.degree. C. Such deterioration decreases the heating efficiency of the silicone oil when sheared by the rotor. This further degrades the heating performance of the heat generator and decreases the efficiency of warming the passenger compartment.
Therefore, the prior art heat generator excites and de-excites the electromagnetic clutch in accordance with the temperature of the engine coolant. Accordingly, the silicone oil in the heat generator is continuously sheared by the rotor until the temperature of the engine coolant reaches the predetermined limit, or de-exciting temperature. However, time is required for the heat of the silicone oil to warm the engine coolant to the de-exciting temperature. Thus, the rotor may continue to rotate at high speeds (e.g., 6,000 rpm to 8,000 rpm) even though the silicone oil has exceeded the recommended maximum heat-generating temperature. This leads to early deterioration of the silicone oil.
To solve this problem, the electromagnetic clutch can be controlled such that it is de-excited when the rotating speed of the rotor or the engine speed exceeds a predetermined fixed threshold value. The rotating speed that heats the silicone oil to its maximum heat-generating temperature is selected as the threshold value.
However, there are cases in which the temperatures of the silicone oil and the engine coolant are both low. In such cases, it is desirable that the silicone oil be heated quickly by increasing the rotating speed of the rotor to produce a large amount of heat within a short period of time. Therefore, if the rotating speed of the rotor is limited to the threshold value, the heat generator may not be able to meet this demand.